Mild cosmetic compositions must satisfy a number of criteria including cleansing power, foaming properties and mildness/low irritancy/good feel with respect to the skin, hair and the ocular mucosae. Skin is made up of several layers of cells which coat and protect the keratin and collagen fibrous proteins that form the skeleton of its structure. The outermost of these layers, referred to as the stratum corneum, is known to be composed of 250 .ANG. protein bundles surrounded by 80 .ANG. thick layers. Hair similarly has a protective outer coating enclosing the hair fibre which is called the cuticle. Anionic surfactants can penetrate the stratum corneum membrane and the cuticle and, by delipidization destroy membrane integrity. This interference with skin and hair protective membranes can lead to a rough skin feel and eye irritation and may eventually permit the surfactant to interact with the keratin and hair proteins creating irritation and loss of barrier and water retention functions.
Ideal cosmetic cleansers should cleanse the skin or hair gently, without defatting and/or drying the hair and skin and without irritating the ocular mucosae or leaving skin taut after frequent use. Most lathering soaps, shower and bath products, shampoos and bars fail in this respect.
Certain synthetic surfactants are known to be mild. However, a major drawback of most mild synthetic surfactant systems when formulated for shampooing or personal cleansing is poor lather performance compared to the highest shampoo and bar soap standards. Thus, surfactants that are among the mildest are marginal in lather. The use of known high sudsing anionic surfactants with lather boosters, on the other hand, can yield acceptable lather volume and quality but at the expense of clinical skin mildness. These two facts make the surfactant selection, the lather and mildness benefit formulation process a delicate balancing act.
In addition to the cleansing and lathering performance attributes desired by consumers it is of particular value that personal cleansing products further deliver certain in-use rheological properties. In particular a shower gel product which is capable of demonstrating shear thinning behaviour during application to the skin is preferred by consumers. It is known that water soluble polymers can be used to provide product thickening attributes and furthermore that hydrophobically modified water soluble polymers can exhibit enhanced product thickening behaviour and impart shear thinning characteristics. However it is also known that such product thickening/shear thinning effects are affected by the total surfactant level present in the system and in fact the thickening/thinning attributes can be significantly diminished in the presence of even very low levels of water soluble surfactants. A secondary effect of high surfactant and electrolyte concentration in systems containing hydrophobically modified water soluble polymers (HMWSPs) is that at increased surfactant levels the HMWSP can become increasingly insoluble in the product matrix.
Thus a need exists for personal cleansing products which deliver acceptable in-use skin feel characteristics but which will not dehydrate the skin or result in loss of skin suppleness, which will provide a level of skin conditioning performance in a wash and rinse-off product which previously has only been provided by a separate post-cleansing cosmetic moisturizer, which demonstrate desirable in-use rheology behaviour and which will produce a foam which is stable and of high quality, which are effective hair and skin cleansers, which have good rinsibility characteristics, and which at the same time have stable product and viscosity characteristics and remain fully stable under long term and stressed temperature storage conditions.
Nonionic water-soluble cellulose ethers are employed in a variety of applications, including hair care compositions. Widely used, commercially-available nonionic cellulose ethers include methyl cellulose, hydroxy propyl methyl cellulose, hydroxyethylcellulose, hydroxypropyl cellulose and ethyl hydroxyethyl cellulose.
Certain modified cellulose ethers have been disclosed in U.S. Pat. No. 4,228,277, Landoll, issued Oct. 14, 1980, which are relatively low molecular weight but which are capable of producing highly viscous aqueous solutions in practical concentrations. These materials are nonionic cellulose ethers having a sufficient degree of nonionic substitution selected from the group consisting of methyl, hydroxyethyl, and hydroxypropyl to cause them to be water-soluble and which are further substituted with a hydrocarbon radical having from 10 to 24 carbon atoms in an amount between 0.2 weight percent and the amount which renders said cellulose ether less than 1%, by weight, soluble in water. The cellulose ether to be modified is preferably one of low to medium molecular weight; i.e., less than 800,000 and preferably between 20,000 and 700,000 (75 to 2500 D.P.).
Modified cellulose ethers have been disclosed for use in a variety of composition types. Landoll ('277) teaches the use of certain materials in shampoo formulations. Hercules trade literature teaches the use of modified cellulose ethers materials in shampoos, liquid soaps, and lotions. U.S. Pat. No. 4,683,004 discloses the use of modified cellusoe ethers in mousse compositions for the hair. U.S. Pat. No. 4,485,089 teaches dentifrice compositions containing modified cellulose ethers.
It has now been found that personal cleansing compositions having improved skin feel and moisturisation attributes, both in use and after use, which deliver desirable thickening/thinning (rheology) benefits and good product stability can be formed by the combination of certain modified cellulose ethers, cationic polymers and surfactants. It has also been found that certain modified cellulose ethers having specified degrees of substitution and chain lengths confer particular benefits in personal cleansing compositions in terms of stability and application characteristics.